Extreme ultra-violet (EUV) light, which is defined as electromagnetic radiation with wavelengths between 124 nm and 10 nm, is used in next-generation photolithography systems to produce structures smaller than is possible with current ultra-violet light sources, such as excimer lasers. However, as EUV light is strongly attenuated by many substances, the path of EUV light from the light source to the location of its use is generally sealed and kept at a very low pressure to minimize attenuation of the EUV light.
The low-pressure environment within a semi-conductor inspection metrology or lithography apparatus using EUV light causes various contaminants to outgas from the walls and other surfaces which comprise the channel around the EUV light path. These contaminants, if not promptly removed, can foul the optics in the EUV light path or accumulate in the light path and result in the attenuation of the EUV light. Therefore, a purge gas is generally introduced into the EUV light path, causing the contaminants to either diffuse into the purge gas or be advected by the purge gas flow, after which, the purge gas and contaminants can be removed from the EUV light path by a vacuum pump.
For example, due to the low pressures found within apparatuses that use EUV light, such as semi-conductor inspection metrology or lithography apparatuses, desorption of contaminants, in the form of outgassing, occurs constantly. Metal surfaces near the optic elements can outgas significant amounts of water (H2O), in the form of water vapor, at ambient temperature. Similarly, mirror adhesives, actuators, cables, and other mechanical elements can release hydrocarbon gases (HC) through gaps surrounding the mirrors. This desorption of contaminants causes various problems within these apparatuses, including degradation of optical performance in optic elements and attenuation of the EUV light via absorption by the contaminants. Optics requirements dictate very low partial pressures of these contaminant gases in the ducts, which may also be referred to as channels, through which the EUV light passes. These requirements can be achieved by using gas purging with the affected apparatus.
Typically, the purge gas is introduced into the EUV light path in a general flow and relies primarily on a high flow rate to remove the contaminants within the semi-conductor inspection metrology or lithography apparatus. However, this undirected, high flow rate approach has several drawbacks. First, extremely large and expensive pumps are needed to reduce the contaminants to acceptable levels, and frequently, these levels cannot even be achieved. Second, by introducing purge gas into the EUV light path in an undirected manner, there is no control over the flow of the purge gas within the semi-conductor inspection metrology or lithography apparatus. Certain regions, such as those that contain mirrors or other optics, require fastidious removal of contaminants so that the optics are not fouled. Finally, because undirected purge gas flow is relatively inefficient at removing contaminants, more purge gas must be introduced into the semi-conductor inspection metrology or lithography apparatus, which tends to attenuate the EUV light traveling through the same.